Flex vs. JavaFX vs. Silverlight

The RIA world has been seeing intensified competition since last year's releases of Microsoft Silverlight and Sun JavaFX,
which joined the Adobe Flex/AIR to become the major RIA development platforms.
Even though all three of them intent to reach the the same goal, richer user experience and more creative media delivering, the technologies behind them are vast different. But how different from a RIA developer's perspective?